The present invention is a new type of dye for use with liquid crystal materials, which provides visible displays having enhanced optical and performance characteristics.
Liquid crystals may be defined as that class of matter which has an intermediate or mesomorphic state in which these substances behave mechanically as liquids yet exhibit many optical properties of crystals. The mesomorphic state or liquid crystalline phase is obtained either by heating liquid crystals that are in the solid phase or by cooling liquid crystals that in the liquid phase. Liquid crystal substances may exist in one of the cholesteric, smectic or nematic state. The cholesteric state is distinguished from the nematic state by its markedly different optical properties. For example, cholesteric materials are optically negative whereas nematic materials are optically positive. Optically positive liquid crystals transmit light more slowly perpendicular to the layers of molecules than parallel to them. Nematic liquid crystals orient a beam of ordinary light into two polarized components whose transverse vibrations are at right angles to each other. However, application of force field, such as an electronic field, lines the molecules up, changing their optical properties.
It has been said that the most popular liquid crystal display devices are of the "twisted" nematic type due to their low voltage and current requirements. Twisted nematic type liquid crystal devices, however, exhibit several shortcomings in use, such as reduced brightness in the reflective mode, restricted viewing angle, increased costs, and low adaptibility to multi-color displays, because of the need for polarizers to produce a contrast display system.
In order to overcome these problems liquid crystal "guest-host" display devices have been developed which eliminate the need for polarizers. The term "host" refers to the liquid crystal material, and the term "guest" refers to an agent which may be aligned by the host to produce contrasting light absorptive states in response to selectively activeable external stimuli. For example, U.S. Pat. No. 3,833,287 to Taylor, et al. discloses a host nematic liquid crystal used in conjunction with a pleochroic guest material which in mixture corresponds to the helical molecular orderings of the nematic liquid crystal material. In the helical ordering the guest material is absorptive of incident light, whereas the guest material is nonabsorptive of light when aligned by the nematic crystals under the influence of an applied electric field. Accordingly, when an electric field is applied to a thin layer of the mixture in the display device, incident light passes through the mixture and is reflected off the backing thus exhibiting the color of the backing.
In recent years refinement of effective guest-host systems have been developed using dichroic dyes. Dichroism is the property whereby an oriented assembly of dye molecules exhibit relatively low absorption of a given wavelength of light in one state of orientation and a relatively high absorption of the same wavelength in another state of orientation with respect to the light source. The orientation can be brought about by dissolution of the dye in a liquid crystal solvent or by embedding the dye in a stretched plastic.
In order to achieve sufficient contrast and the high brightness required for colored and multi-colored display systems, dichroic dyes must be used which have good solubility and high order parameter.
Solubility must be sufficiently high so that thin layers, for example, of twelve micrometers, have adequate light absorption in one of the oriented states. Ionic dyes will generally be inadequate not only because of their low solubility but also because they increase the electrical conductivity of liquid crystals.
Order parameter is a quantitative measure of the degree of molecular order of alignment in a given system. The contrast and brightness are both related to the order parameter S of the dye, where S=(R-1)/(R+2) and R is the ratio of the light absorption in the dye measured with a polarizer respectively parallel and perpendicular to the nematic direction of the liquid crystal host at the wavelength of maximum absorption. High order parameter is promoted by dyes with elongated shape having a large ratio of molecular length to breadth, similar to the shape of the molecules of liquid crystal host material. To insure an elongated shape, the molecules should have a rigid structure which can be obtained, for example, by linking benzene or heterocyclic rings with double bonded groups.
Efforts to provide a dye capable of the abovedescribed performance characteristics have resulted in developments disclosed in a series of patents of Cole Jr., et al., assigned to General Electric Company, in which a number of dichroic liquid crystal systems are shown. In U.S. Pat. No. 4,122,027 of Cole Jr., et al. a tris-azo dichroic dye is described having an order parameter S of 0.70 which has the following general formula ##STR2## wherein R is NHCH.sub.3, NHC.sub.2 H.sub.5, NH.sub.2 or together with R' is ##STR3## U.S. Pat. No. 4,179,395 of Cole Jr., et al. discloses a dichroic liquid crystal composition which includes a pentazo dichroic dye having an order parameter S of at least 0.65, and U.S. Pat. No. 4,128,497 of Cole Jr., et al., again shows tris-azo compounds useful in liquid crystal display systems because of their high order parameter and contrast characteristics.
Other disclosures in the field of utilization of dyes in conjunction with liquid crystal display systems include U.S. Pat. No. 4,137,193 to Osman, et al. and U.S. Pat. No. 4,145,114 to Coates, et al. Osman et al. describe an azo dye guest component having an order parameter of at least 0.65 and an absorption maximum in the range of from 350 to 700 nanometers. The compounds shown in the Osman, et al. '193 disclosure include only one or at most two azo groups. Coates, et al. show pleochroic dye compounds suitable for use in solution with liquid crystal material which may include from one to nine azo groups.
In all the disclosures described above, the desired effect of the technical development is centered on creating a liquid crystal display system having good contrast and high order parameter as well creation of dyes with good solubility for ease of operation. In furtherance of liquid crystal display technology the present invention provides a new family of dichroic dyes having good contrast, high order parameter, and very high stability to ultraviolet/visible light.